1. Field of the Invention
The present invention relates to printers and printing systems and, more particularly to controlling and matching the printing characteristics of tandem print engines of a printing system.
2. Description of the Related Art
Various printers and printing systems use more than one print engine, where the print engines are typically arranged in tandem. A typical high speed duplex printing system uses two print engines. Each of the two print engines prints to one side of the print medium fed through the printing system. The two print engines are either each orientated within the printing system for printing on one side of the print medium and/or there is a mechanism for manipulating the print medium such that each side thereof is presented to the two print engines for printing. In this manner, the two print engines may efficiently and quickly produce a duplex print job (i.e., double sided copy) on the print medium. Ideally, the optical density (i.e., the darkness) of the duplex print job is the same on both sides of the print medium. However, a problem can result when the optical density of the image printed on one side of the print medium does not match the density of the image printed on the other side of the print medium.
Though generally known, a brief discussion of the typical duplex printer process will be explained to highlight some of the causes for print mismatches between print engines in a duplex printing system. In an electrophotographic (EP) print engine, a photoconductive drum is charged to a uniform potential whereupon light is then selectively transmitted in order to write a latent image of the print job image onto the photoconductive drum surface. The latent image is then developed by depositing, typically, a mixture of toner particles and triboelectrically charged carrier granules on the photoconductive drum. Due to differences in electrical potentials, the toner particles are attracted to the latent image formed on the photoconductive drum and thus form a latent image of toner particles on the drum. The toner particles, arranged in the form of the latent image, are then transferred to a print medium. The print medium is next heated to fuse the image onto the print medium. The print medium is then inverted for duplex printing. The print medium is then fed to the second print engine in the duplex printing system so that the process may be repeated for the second engine, whereby an image is printed on the second side of the print medium.
While the two print engines used in a duplex printer are typically identical to each other, there often exists differences in the print outputs produced by each of the two print engines. The differences in the print outputs from the two print engines can be attributed to differences between the print engines and the effect of the printing process on the print medium. Although the print engines are typically the same (i.e., same model number, etc.), differences in the print engine print outputs can still occur due to manufacturing differences and degradation of the print engines during use, both of which can impact performance of the print engines. Adjusting the print engines so that they each operate within the printer manufacturer""s tolerances does not assure that the print output of each print engine will match the other.
For example, the printing process can introduce changes to the amount of toner transferred to the print medium due to heating of the print medium, due to variables in the transfer of the image to the print medium, and due to the spatial separation of the print engines and environmental changes experienced by the print medium as it is advanced through the printing system. These, and other influences, can affect the print output of a print engine in a printing system such the print output of one print engine in a printing system does not match the print output of another print engine in the printing system, i.e., a printing mismatch.
A printing mismatch between the print engines of a printing system is often manifested in density differences in the print job produced by a duplex printing system wherein the density of the image printed by one print engine differs from the density of the image produced by the other print engine. Compensation of printing mismatches can be applied using PostScript(trademark) transfer functions, provided the data to be printed is contone. Binary data cannot be corrected using PostScript(trademark) transfer functions. The printing of binary data and binary text is the focus of the present invention. Note however that after the print engines have been adjusted so that binary image and text printed by the print engines matches, it follows that contone data RIPed to binary data (i.e., using Raster Image Processing to convert data to binary data) using a single transfer function also matches when printed by the adjusted print engines.
Some previous printing systems have attempted to compensate for print mismatches in printing systems using tandem print engines by regulating the consistency of the density of the image to be printed. These previous systems however, measured the density of patches formed on the drum or transfer members during the EP process. These systems have the disadvantage of not accounting for the influences of the printing process and the transfer characteristics of the print engines on the print medium, factors that can cause changes to the resultant print job.
A printing system described in U.S. patent application Ser. No. 08/865,039, now U.S. Pat. No. 6,147,698 measures the density of the various colors printed by a color electrostatic printing system on a print medium and is controlled to adjust the print stations of the printer to bring the print stations within printer tolerances.
Accordingly it is an object of the present invention to provide a method and system for achieving matching print outputs from the print engines of the printing system.
It is another object of the present invention to provide a method and system for controlling the print engines of a printing system that measures the density of a print output on the print medium.
It is further object of the present invention to provide a method and system for controlling the print engines of a printing system wherein the density of the print outputs by the print engines is measured after the print medium has advanced substantially through the entire printing system.
It is another object of the present invention to provide a method and system for controlling the print engines of a printing system wherein each print engine produces print outputs of a consistent density.
It is still another object of the present invention to provide a method and system for controlling the print engines of a printing system wherein the print engines of the print system can be independently adjusted to achieve a desired print output density.
It is yet another objective of the present invention to provide a method and system for controlling the print engines of a printing system wherein the density of the print output produced is controlled preferably by adjusting the contrast and PQE (Print Quality Enhancement)boldness of the print engines.
It is a further object still of the present invention to provide a method and system for controlling the print engines of a printing system wherein the density of the print outputs from the print engines are efficiently and effectively matched by varying the factors that most influence the density of the print outputs.
A duplex printing system including two print engines arranged in tandem for printing a density patch on a print medium, at least one density sensing device adapted for measuring the density of the density patch printed on the print medium by the two print engines and a control means for controlling the adjustment of the print engines to enable the density of the print output of the first and the second print engines to match. The printing system of the present invention is further characterized by the measurement of the density of the density patch printed by each print engine after the print medium has advanced through the printing system. Thus, the print medium and density patch printed by each print engine is subjected to the same conditions and influences as a print job processed by the printing system. Accordingly, density measurements and adjustments made to the print engines to vary the density of the print engine outputs will be realized in a print job subsequently produced by the printing system and the resultant print job will comprise printed images printed by two print engines that have matching print characteristics, namely matching densities.
Although discussed herein primarily in the context of a printing system adapted for printing duplex print jobs, the printing system of the present invention is not limited to this, or any other, particular embodiment or application. As will be seen from the following detailed discussion, the system and methods of the invention are adaptable to printers, printing systems, copiers and other document producing devices having two print engines. The inventions is also applicable to printing systems and print jobs distributed over a network where the matching of the print output characteristics is also desirable.